In recent years, equipments for information terminals and the like have been highly developed and, accordingly, it has been more and more difficult to manipulate such a highly developed equipment. For example, the number of manipulation buttons is increased and a density is increased, so that an “operation error”, i.e., an error of mistakenly pressing a manipulation button by a user easily happens. Also, in order to complete an operation within a limited time, a user is required to complete manipulation in a short time in many situations and, therefore, an operation error such as mistakenly pressing a button inevitably occurs.
To eliminate such operation errors, modification of interface design such as changes of GUI, adjustment of button position and the like has been conventionally examined. For example, in Patent Reference 1, an equipment including an All Clear button for resetting a function which has been set to an initial state is disclosed. In the equipment, when the All Clear function is instructed, a display for confirming whether or not the All Clear function may be executed appears and, after a confirmation instruction is received, the All Clear function is executed. Referring to Patent Reference 2, in a car navigation system with which both of an audio compact disk and a map storage compact disk can be used, when a map storage compact disk is loaded in the car navigation system and a user performs an eject manipulation of the compact disk, a display for operation. confirmation is performed.
In the above-described examples, for an operation which largely changes performance of an equipment, such as All Clear of setting function (Patent Reference 1), ejection of a map storage compact disk (Patent Reference 2) and the like, which makes map display impossible, confirmation of a user for an operation instruction is requested and only if confirmation is obtained, the function is executed.
In the same manner, in operation of computer, before an irreversible, important operation such as deletion of a file, format of a hard disk and the like is performed, a confirmation display appears for selecting Yes or No.
Unlike the above-described example where an operation of confirming user's intention for such an important operation is requested, there is another example, a state of a user is recognized by measuring a biological signal of the user. In Patent Reference 3, at an interface of computer or the like, the degree of fatigue of a user is judged from a heart rate, myoelectric potential, the degree of sweating and the like. When the user is fatigue, a screen display is enlarged, contrast is emphasized, sound is turned to louder, or an input sensitivity of an input device such as a mouse and a pen tablet is increased. Thus, it is believed that the degree of fatigue of the user can be reduced and, as a secondary effect, reduction in operation error can be expected.    Patent Reference 1: Japanese Patent No. 3060949    Patent Reference 2: Japanese Utility Model No. 2578013    Patent Reference 3: Japanese Laid-Open Publication No. 2001-184139